The present invention relates generally to antennas used in mobile and/or military applications. More particularly, the present invention relates to a broad band antenna that provides an instantaneous bandwidth of about 482 Megahertz(MHz) between 30-512 MHz with a relatively low voltage standing wave ratio (VSWR) and high gain. Specifically, the present invention provides a monopole broad band antenna with a series of inductor-resistor networks which effectively change the electrical length of the antenna.
It is known that electromagnetic communication systems employ broad bandwidth techniques, such as the so-called frequency-agile or frequency-hopping systems in which both the transmitter and receiver rapidly and frequently change communication frequencies within a broad frequency spectrum in a manner known to both units. When operating with such systems, antennas having multiple matching and/or tuning circuits must be switched, whether manually or electronically, with the instantaneous frequency used for communications. As such, it is imperative to have a single antenna reasonably matched and tuned to all frequencies throughout the broad frequency spectrum of interest. Although the art discloses such broad band antennas, such as exemplified in U.S. Pat. No. 4,958,164 and U.S. application Ser. No. 09/175,008, which are owned by Assignee of the present invention, these antennas provide a somewhat limited frequency range.
As is well known in the art, a thin linear monopole antenna is normally used in a manner that requires its electrical length to be a quarter wavelength or 90 electrical degrees. These antennas require a ground plane, which is a large plane of sheet metal, such as a car or vehicle body made of metal, to provide the other half of the antenna. Therefore, the characteristics of the ground dependent xe2x80x9cquarter wavexe2x80x9d antenna are well known.
In order to enable a thin linear monopole antenna to be multi-band, the known art teaches placement of xe2x80x9ctraps,xe2x80x9d which are parallel inductors and capacitors, at various places in series with thin linear radiators (conductors). Such a construction results in a monopole that can be used for several frequencies or very narrow bands of frequencies. Unfortunately, the useful bandwidths for this type of antenna are very narrow, usually on the order of KHz or 2-3 MHz. With this in mind, it would be presumed that additional traps in series at various points with the linear radiators should produce additional bandwidth. However, the number of traps is usually limited to 2 or 3. The reason for this is that adjustment of each trap to its specific frequency or operational bandwidth is interdependent on the adjustment of all the traps within the antenna.
The main purpose of utilizing a trap is to change the electrical length of the monopole radiator as the frequency of operation is changed. Moreover, at a specific trap""s operational frequency or bandwidth, the current in the linear radiator physically above the trap in question, is reduced to or near zero so that the current distribution of the radiator physically below the trap in question is approximately that of a quarter-wave monopole radiator. In view of the interdependency of each trap in order to obtain a desired frequency bandwidth, there is currently not available in the art a linear monopole antenna with a bandwidth anywhere near 482 MHz. Nor is there available an antenna with such a wide bandwidth that also has a relatively low VSWR across the bandwidth.
It is thus an object of the present invention to provide a low profile, broad band monopole antenna for frequencies ranging between 30-512 MHz.
It is another object of the present invention to provide an antenna, as above, that operates within the desired bandwidth and requires no field adjustments of any kind that require tuning or power matching of the antenna at any specific frequency within the band of operation.
It is a further object of the present invention, as above, to provide an antenna that when measured on a standard reference 10xe2x80x2xc3x9710xe2x80x2 metal ground plane placed 10xe2x80x2 above the earth ground, does not exceed a VSWR of 4:1 across the very high frequency (VHF) band (30-108 MHz) and does not exceed a VSWR of 3:1 across the ultra high frequency (UHF) band from 108-512 MHz.
It is yet another object of the present invention, as set forth above, to provide an antenna which provides a line of sight, E-field radiation pattern that when measured on a standard metal ground plane placed 10xe2x80x2 above earth ground and compared to resonant quarter wave radiators at any specific frequency within the 30-512 MHz bandwidth, has an E-field gain that is not less than (xe2x88x9211 db) below the reference quarter wave antenna.
It is yet another object of the present invention to provide an antenna, as set forth above, which provides high voltage protection of up to 20 KV rms 60Hz for personnel and equipment should a tip portion of a vehicle-mounted antenna""s radiator come in contact with high voltage power lines.
It is still another object of the present invention to provide an antenna, as set forth above, which provides a transmission line connected to the appropriate transmit/receive equipment and which is enclosed within a flexible spring assembly.
It is still a further object of the present invention to provide an antenna, as above, with a base radiator and a tip radiator enclosed in a reinforced housing that is secured to the spring assembly.
It is an additional object of the present invention to provide an antenna, as above, with an unun transformer within the base radiator to transform the feed point of impedance of the antenna to impedances that meet the VSWR requirements.
It is still yet another object of the present invention, as above, to connect the unun transformer to a linear radiator and a parallel inductor-resistor network to assist in regulating the effective electrical length of the antenna.
It is yet another object of the present invention, as above, to connect the first inductor-resistor network to an additional linear radiator and a second parallel inductor-resistor network to further assist in adjusting the effective electrical length of the antenna.
It is still yet another object of the present invention, as above, to provide the antenna with a tip radiator that is connected to the second inductor-resistor network through a tip capacitor.
The foregoing and other objects of the present invention, which shall become apparent as the detailed description proceeds, are achieved by an antenna operable over a predetermined range of frequency, comprising a transmission line, a transformer network connected to one end of the transmission line, and at least one inductor-resistor network connected to an opposite end of the transformer network, at least one inductor-resistor network changing the effective electrical length of the antenna such that as the frequency of operation changes, the current distribution above and below the inductor-resistor network changes in a corresponding manner.
Other aspects of the present invention are attained by an antenna operable over a predetermined broad band and connected to a transmission line, comprising a tip radiator having a series capacitance, a base radiator connected at one to the tip radiator and at the other end to the transmission line, the base radiator changing the effective electrical length of the antenna such that as the frequency of operation changes, the current distribution along a length of the base radiator changes.
These and other objects of the present invention, as well as the advantages thereof over existing prior art forms, which will become apparent from the description to follow, are accomplished by the improvements hereinafter described and claimed.